The subject invention is directed to a process for producing briquettes from the fines produced by metallurgical processes wherein less than 3% by weight of the briquette formed, is the binder material. Also disclosed is a process description comprising proper sequencing and juxtapositioning of various apparatus for producing such briquettes.
Solids handling is a major part of a variety of processes, especially metallurgical processes. Size degradation inevitably occurs during handling, resulting in the creation of "fines" (a generic term generally used for particles that are smaller than 1/4"). Fines are produced in a variety of other ways as well. Due to the small particle size which vitiates recoveries, and associated problems with handling, fines in the unagglomerated state cannot be remuneratively used. Many metallurgical processes today are generating ultrafine baghouse dusts that are listed as hazardous wastes by EPA, and must be disposed of at considerable cost. These costs are going to be higher as the number of disposal sites shrinks and regulations become even more demanding. Consequently, there is great interest in the industry to somehow avert these disposal costs and perhaps reduce the waste streams through recycling. The baghouse dusts, because of their ultrafine size, are not amenable to briquetting by the standard methods available to date. This invention will relate a way of briquetting these fines, among other things.
Agglomeration of fines to make them more usable has been a common practice for more than 100 years. One method of agglomeration is to use binders. The most commonly used binders include sodium silicate, a lime and molasses combination, Portland cement and water, and steric acid, among others. These binders typically make up about 10% by weight of the final product; and agglomeration is generally a batch process.
Briefly, binder components and the fines are delivered into typical mixing equipment and the mixture is stirred together for a certain length of time to produce a homogeneous mix. No particular attention is paid to the particle size of the binder components as they are delivered into the fines. However, it is known that the surface area of the binder should ideally be greater than or equal to the surface area of the fines. Thus, any binding process involves a basic surface area balance. Since the binder is a relatively minor part of the agglomerate by weight, it becomes clear that to create the same surface area out of the binder as that of a much larger amount by weight, of particulates, is quite a challenge. The typical way of delivering the binder to the mix necessarily requires that a considerable amount of binder be used.
But one cannot keep adding the binder indefinitely. After all, there is a common sense upper limit on the binder percentage for the mixture to be economically viable. The process, due to the batch nature, is forced to stop at a certain binder percentage which in most cases is not really enough to produce a high quality agglomerate. Consequently, typical binder systems produce an agglomerate with poor green strength (i.e. the strength necessary to hold the agglomerate in a given form or shape after partial curing) which is insufficient for maintaining that form or shape during handling and transportation of the resulting briquettes. Typically, such agglomerates are post cured with heat, followed by a curing time of about 24 hours before the agglomerates can be handled or transported. Thus, storage must be made available to accommodate the agglomerates during this long cure period. When such agglomerates are eventually used in the process, high binder content results in excessive energy costs to break the binder down and the inorganic binder residue becomes an impurity.
Some three-part organic binder systems, including one that claims to use a relatively low binder percentage compared to the standard inorganic binder, have recently been offered. However, these systems use lead based catalysts leading to high lead content and also a high level of free formaldehyde, both of which are considered to be hazardous to health. These systems are known to contain a solvent package that poses potential health and environmental hazards as well. Further, these systems comprise a high viscosity resin (over 250 cps) impeding the resin's ability to produce small enough particles that are necessary to achieve the required surface area balance. For these reasons, these binder systems would be unattractive to the Industry.
The following invention relates to bonding fine particles and agglomerating these particles on a continuous basis by using an uncommonly low quantity of binder, i.e. less than about 3%. The binder is organic in nature and can burn off completely, leaving no residue. The low quantity of binder substantially maintains the original chemistries, i.e. the percentage breakdown of most or all of the components or elements in the original fines is almost unchanged. Also, the low quantity of binder saves handling and transportation costs, and saves on energy costs as the agglomerated material is economical in various energy consuming processes.
Therefore, it is an object of this invention to provide a means by which metallurgical fines can be agglomerated with lesser amounts of binder while at the same time producing an agglomerate with sufficient strength to undergo handling, transportation, and use without untimely degradation of the material.
It is a further object of the invention to provide a binder system, devoid of carcinogens or other potentially hazardous components, for the agglomeration of fines.
In addition, this invention will present a way of agglomerating ultrafine dusts such as electric arc furnace dust or basic oxygen furnace baghouse dust, among others.
Finally, it is an object of the invention to provide a continuous agglomeration process which avoids the need for lengthy cure times and thus for holding or storage accommodation.